Doesn't quite work like that though - maximum alternator capacity will only be reached at certain RPM or above - tick over is usually only sufficient to output about 1/3 the alternators maximum plated capacity.
So if you add big electrical loads the alternator will not keep up at idle, you need to be driving which is why many people go for twin alternators and dual batteries.
You don't get long out of an alternator running it at or near fully capacity, 150A is pretty serious at 14.4V is 2.2Kw. The voltage regulator is going to be HOT and the windings will be cooking. A duff battery can kill an alternator fairly quickly due to increased alt loading, so intentionally loading an alt with large loads can do the same - also note your battery will become a weak spot - the battery acts like a big buffer, if you run large loads and the vehicle slows or idles the battery is doing more work as the alt output drops - then you start to drive faster again and the large load coupled with the additional current to recharge the battery can then push the load up significantly.
Note it says 85/150A - that would suggest there are two options on these, probably varied by packages.
I'd work out what I wanted to fit then go from there. Always assume the factory charging and power system is not really up to much more than intended bar a few extras.
